The advent of solid state power drivers has created the need to monitor or to sense load current for device protection and other functions. The power delivery circuits for supplying load current to a load and which further include current sensing capabilities are generally known and are of diverse types. In U.S. Pat. No. 4,654,568 to Mansmann, there is disclosed an H-bridge switch circuit for a load such as a motor. More specifically, the various switches of the H-bridge are MOSFET semiconductors, with two of those MOSFETs comprising current sensing MOSFETs. In that described embodiment, the current sensing MOSFETs are connected in the "low" side of the power delivery circuit to the motor. The current sensing MOSFETs are multi-cellular devices in which a major current-carrying cellular portion consists of a large number of cells connected in parallel for carrying the main load current and in which a minor current-carrying cellular portion consists of a relatively small number of cells substantially in parallel therewith and having a separate terminal for carrying a current proportional to that load current. That separate terminal is termed the "current sensing" terminal and may be useful for developing a signal indicative of the major current carried by the current sensing MOSFET.
In U.S. Pat. No. 4,654,568, the current conducted via the current sensing terminal is developed as a voltage level which is applied to an input of an operational amplifier for amplification and subsequent evaluation. The current sensing circuit which converts the current carried at the current sensing terminal to a corresponding voltage level appears in the patent as at least one sensing resistor operatively connected between the current sensing terminal and ground. The operational amplifier is connected to the current sense terminal and amplifies the voltage developed across that resistor (sense voltage). Sensing current in this manner inherently induces errors in the ratioing of the majority current and the sensing current in the current sense MOSFET. More significantly, as the temperature of the current sense MOSFET increases, as under increasing load conditions, the aforementioned induced error percentage and likewise associated sense voltage error percentage increases.
Furthermore, for safety reasons required in automotive applications, power switch protection and current sensing need to be implemented in a "high side" configuration. This configuration places the current switch or power switch in between the positive supply and the load. If the current sensing method demonstrated in U.S. Pat No. 4,654,568 were to be used in a high side configuration, it would have all of the previously described problems. Further, the operational amplifier supply voltage would have to be higher than the sum of the current sense terminal voltage and the amplified voltage drop across the sensing resistor. Since the voltage drops across the current sense MOSFETs (power MOSFETs) are very low, this would required an operational amplifier voltage supply to be higher than the voltage supply connected to the current sense MOSFET, which is typically the battery, V.sub.BATT, in automotive applications. This method is further complicated by requiring the threshold reference voltage, which needs to be generated and used for comparing the amplified resistor voltage drop to some threshold for protection, to be operatively referenced above the current sense terminal potential, which is changing with varying load conditions and battery voltage.
Alternatively, an operational amplifier could be connected in a current amplifier configuration, sometimes called a virtual ground reference, instead of a voltage amplifier configuration. This would require the operational amplifier to have a high common mode input voltage nearly equal to the supply voltage, a threshold reference voltage to be operatively referenced below the current sense terminal potential which is changing with varying load conditions and battery voltage, and would require an operational amplifier to work at the high voltage transients that are present in automotive applications (typically filtered to 45 volts).